1. Field of the Invention
The invention relates to an interference cancellation method in a radio system.
2. Description of the Related Art
In mobile systems, information is transferred between a mobile network and a mobile station by means of data transmission resources. The data transmission resources of a mobile network are specified in different manners depending on the multiple access method of the system. In radio networks employing the frequency division multiple access method (FDMA), users are distinguished from each other based on the frequency used. In radio networks employing the time division multiple access method (TDMA), several users are able to communicate in the same frequency band, in which the users are temporally distinguished from each other by the division of the information transmitted or received by the users into timeslots. In radio systems using the code division multiple access method (CDMA), several transmitting and receiving stations communicate in the same radio spectrum frequency band simultaneously. For the duration of the connection, a spreading code is reserved for each user, with which the user spreads the information included in a baseband signal. The receiver of the signal, in turn, is able to identify the information transmitted by the user by despreading it using the spreading code. A radio system may also be implemented by combining multiple access methods, for instance in a hybrid system based on the TDMA and CDMA multiple access methods, users communicating in each timeslot are distinguished from each other by means of spreading codes.
None of the above described multiple access methods guarantees an ideal and noise-free radio connection between users and a mobile network. For instance in the TDMA system, users communicating in adjacent timeslots and adjacent cells of the mobile system interfere with each other. The drawback in the CDMA system, in turn, is that the users operating in the same frequency band interfere with each other's transmissions due to cross correlation between spreading codes. In addition to the interference caused by users to each other, the signal is distorted on the radio path due to the shapes of the surrounding terrain, for example. Multipath propagation refers to a user signal being reflected from several different targets when propagating, thus generating several components, delayed in different manners, from the same signal in a receiver. Multipath-propagated components of a signal may be subjected to fading for instance when the signal is reflected from two objects that are close to one another. When fading is significant, the signal cannot be received at all. Interference in a radio transmission may also be caused by another radio system operating in an adjacent or even the same frequency band, which is due to an increasing number of users and the subsequent more effective utilization of the frequency ranges. An example of such a solution is placing second and third generation mobile networks in the same frequency band.
Placing mobile networks in the same frequency band often means that the desired signal has to be received in a very noisy environment. For instance in a radio system employing the CDMA multiple access method, a RAKE receiver based on reception via at least one antenna is used in a base station. In this case it is to be expected that fadings in different antennas do not correlate with each other. In a RAKE type of CDMA receiver, multipath propagation may be utilized by receiving several components, delayed in different manners, and combining them to obtain the best identification of a user signal. A RAKE receiver is composed of correlation branches (fingers), each of which receives a multipath-propagated component. The impulse response may be measured for instance by means of a matched filter (MF), from which a delay profile may be generated. A matched filter is used on information received for instance on a pilot channel or in the pilot sequence of a radio burst. Pilot symbols are a number of symbols known to the receiver and the transmitters, allowing the receiver to generate an estimate of the quality of the radio channel used. A matched filter calculates the convolution between the received signal and the spreading code for instance at intervals of ½ chip. In this way an impulse response pattern is generated for the multipath-propagated components of the received radio channel, the pattern including information on the signal power and the delays of the multipath-propagated components.
Several methods are known for combining signals received via different antennas, such as the IRC (interference rejection combining) method and the MRC (maximum ratio combining) methods. In IRC, the signals received via different antennas are weighted by complex weighting coefficients that are set based on the spatial characteristics of the received interfering signal. In the MRC method, the weighting coefficients are selected such that the signal-to-noise ratio, i.e. the power ratio between the desired signal and the interfering signal, is maximized. In a known method, the MMSE (minimum mean squared error) method is used in selecting the weighting coefficients, and the differences between the training sequence received in the signal and the bit sequence received in the receiver are compared, and said differences are minimized.
The performance of prior art solutions for combining signals received via different antennas is not sufficient. They are also difficult to implement in a receiver.